Method and a system for evaluating the physical consumption of a polishing pad of a cmp apparatus, and cmp apparatus

ABSTRACT

A method for evaluating the physical consumption of a polishing pad of a CMP apparatus provided with an eddy current sensor that is configured to sense a distance with a substrate on which a layer to be processed extends. The method includes: generating a magnetic field adapted to induce eddy currents in the layer; acquiring an eddy current signal generated by the layer in response to the magnetic field; calculating an average value of the eddy current signal in the initial time frame; comparing the average value with a pre-set threshold value; and determining a maintenance condition of the polishing pad based on a result of the comparison.

PRIORITY CLAIM

This application claims the priority benefit of Italian Application forPatent No. 102020000015790, filed on Jun. 30, 2020, the content of whichis hereby incorporated by reference in its entirety to the maximumextent allowable by law.

TECHNICAL FIELD

This disclosure relates to a method and a system for evaluating thephysical consumption of a polishing pad of a Chemical MechanicalPolishing (CMP), apparatus. This disclosure further relates to a CMPapparatus including this system.

BACKGROUND

Integrated circuits are manufactured by sequential steps of depositionand partial removal of layers on a substrate of a silicon wafer; suchlayers may either be conductive, semiconductive or insulating. Onemanufacturing step involves depositing a conductive (typically,metallic) filler layer and then planarizing the filler layer. Chemicalmechanical polishing (CMP) is a key step in the modern semiconductormanufacturing processes to achieve planarization of layers. For example,to form vias or plugs, one technique is to deposit a conductive fillerlayer (typically, made of Copper) over a pre-patterned barrier layer(e.g., an insulating film), which presents trenches or holes, extendingover the substrate, with the aim of filling the trenches/holes by theconductive filler layer. With a CMP process, the conductive filler layeris removed until the barrier layer is reached and exposed. After theCMP, the portions of the conductive layer remaining within the trenchesin the barrier layer effectively form conductive vias, or plugs thatprovide vertical conductive paths for signal routing on the substrate orother uses. The CMP can be used for planarization steps that may beutilized to planarize the wafer for, for example, furtherphotolithography steps.

During CMP, the wafer is mounted on a carrier or polishing head. Thesurface of the wafer to be planarized is placed against a rotatingpolishing pad having a roughened surface. The carrier head rotates thewafer while pushing it against the polishing pad. Usually, a polishingslurry (including at least one chemically-reactive agent and abrasiveparticles) is supplied to the surface of the polishing pad.

The endpoint condition of the polishing process can be achieved indifferent ways, depending on the material that is removed or the statingcondition.

The typical ones are optical endpoints, where there is a measure of thereflected light intensity (used when a metal film is removed until anoxide layer is reached) or an interferometric measurements in the caseof solely oxide layers, or Eddy current method, which is anon-destructive method typically used in CMP of Copper layers (detectionof the end point is achieved by measuring the variations of the currentinduced the thin metallic layer that is on top on the wafer prior CMP).Specifically, the eddy current sensor operates based on the inductiveeddy-current principle. It measures the distance based on the extractionof energy from an oscillating circuit, which is used to generate eddycurrent in an electrically-conductive materials. When the sensing coilis supplied with an alternating current, it causes a magnetic field toform around the coil. If an electrically conducting material is placedin this field, an eddy current field is induced according to theFaraday's induction law. When the object moves, it causes the change inthe impedance of the coil, which is proportional to the change in thedistance between the sensor and the target.

FIG. 1 is a simplified schematic diagram of the principle upon which aneddy current sensor operates. An alternating current iac flows through acoil 1 in close proximity to a conductive layer 2 extending on asubstrate 5 of a wafer 3. In a CMP apparatus, the coil 1 is fixed orotherwise coupled to the polishing pad (not shown). The alternatingcurrent iac is made to flow through the coil 1, thereby causing thegeneration of an electromagnetic field. In FIG. 1, the conductive layer2 represents the above-mentioned conductive layer (Copper film), thethickness of which has to be reduced (and measured) during CMP. Theelectromagnetic field generated by the coil 1 induces eddy currents 4 inconductive layer 2. The eddy currents, in turn, have an impact on theimpedance of the coil 1. Since, during the CMP, the polishing pad ispressed against the conductive layer 2, the effect of eddy currents 4 oncoil 1 is a function of a distance 6 between the conductive layer 2 andthe coil 1. Therefore, the reduction of the distance 6 caused by athickness reduction of the conductive layer 2 can be measured bymeasuring the impedance change of the coil 1.

One major issue of the CMP process is the evaluation of the actualconsumption of the polishing pad of the CMP apparatus, which is ofinterest because the overconsumption of the pad can lead tooverpolishing or underpolishing. Currently, the lifetime of eachpolishing pad is estimated based on experience/working hours, so thatthe polishing pad is replaced after a predetermined number of workinghours, regardless of the actual consumption of the polishing pad.

That means that a polishing pad may be replaced well before completeconsumption (thus increasing the costs), and another polishing pad maybe replaced after complete consumption (thus compromising the polishingstep). Both cases should be avoided.

A procedure to automatically check the consumption of the pad istherefore desirable.

SUMMARY

The aim of the present disclosure is to provide a method and a systemfor evaluating the physical consumption of a polishing pad of a CMPapparatus, and a CMP apparatus including such system, that overcome thedrawbacks mentioned above.

According to this disclosure, a method and a system for evaluating thephysical consumption of a polishing pad of a CMP apparatus, and a CMPapparatus including such system.

For example, disclosed herein is a method for evaluating physicalconsumption of a polishing pad of a Chemical Mechanical Polishing (CMP)apparatus, where the CMP apparatus is provided with an eddy currentsensor that is configured to sense a distance using a substrate on whicha layer to be processed by the CMP apparatus extends. This methodincludes: generating, using the eddy current sensor during an initialtime frame of CMP processing of the layer, a magnetic field configuredto induce eddy currents in the layer; acquiring, using the eddy currentsensor during the initial time frame, an eddy current signal generatedby the layer in response to the magnetic field; calculating an averagevalue of the eddy current signal in the initial time frame; comparingthe average value with a pre-set threshold value; and determining amaintenance condition of the polishing pad based on a result of thecomparison.

The maintenance condition may be desire for replacement of the polishingpad.

The initial time frame may be such that the eddy current signalgenerated by the layer during the initial time frame is saturated and/orsubstantially time-invariant.

The initial time frame may be between 0 and 70 seconds of the CMPprocessing of the layer.

The initial time frame may be between 0 and 20 seconds of the CMPprocessing of the layer.

The eddy current sensor may be arranged with respect to the polishingpad in such a way that the eddy current signal is a function of both athickness of the layer and a thickness of the polishing pad.

The eddy current sensor may be supported by, and move with, a supportingplaten over which the polishing pad is coupled.

The method may further include arranging the layer against the polishingpad before the initial time frame of processing of the layer by the CMPapparatus.

Also disclosed herein is a system for evaluating physical consumption ofa polishing pad of a Chemical Mechanical Polishing (CMP) apparatus. Thesystem includes: an eddy current sensor configured to sense a distanceusing a substrate on which a layer to be processed by the CMP apparatusextends, wherein the eddy current sensor is configured to generate,during an initial time frame of CMP processing of the layer, a magneticfield configured to induce eddy currents in the layer and to acquire,during the initial time frame, an eddy current signal generated by thelayer in response to the magnetic field; and processing circuitrycoupled to the eddy current sensor. The processing circuitry mayconfigured to: calculate an average value of the eddy current signal inthe initial time frame, compare the average value with a pre-setthreshold value, and determine a maintenance condition of the polishingpad based on a result of the comparison.

The maintenance condition may be a replacement of the polishing pad.

The initial time frame may be such that the eddy current signalgenerated by the layer during the initial time frame is saturated and/orsubstantially time-invariant.

The initial time frame may be between 0 and 70 seconds of said CMPprocessing of the layer.

The initial time frame may be between 0 and 20 seconds of said CMPprocessing of the layer.

The eddy current sensor may be arranged with respect to the polishingpad in such a way that the eddy current signal is a function of athickness of the layer plus a thickness of the polishing pad.

The eddy current sensor may be supported by, and move with, a supportingplate over which the polishing pad is coupled.

Also disclosed herein is a chemical machine polishing (CMP) apparatus,including: a wafer carrier facing a polishing pad, the polishing padbeing disposed over a supporting plate; a wafer carried by the wafercarrier, the wafer having a top surface over which a conductive layerextends; and an eddy current sensing device configured to induce an eddycurrent field in the conductive layer, and detect a change in impedanceof a coil of the eddy current sensing device; wherein the eddy currentsensing device is further configured to generate, from the change inimpedance, an output signal based upon a thickness of the conductivelayer and a distance between the eddy current sensing device and theconductive layer, the output signal corresponding to a thickness of thepolishing pad.

The eddy current sensing device may be supported by, and move with, thesupporting plate.

The eddy current sensing device may induce the eddy current field in theconductive layer by an alternating current flowing through the coil.

The change in the impedance may indicate that a thickness of thepolishing pad has reduced below a threshold level.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, some embodiments thereof will now bedescribed, purely by way of non-limiting example and with reference tothe attached drawings, wherein:

FIG. 1 shows a schematic implementation of an eddy current sensoroperatively coupled to a wafer, according to the prior art;

FIG. 2 shows a schematic implementation of a CMP apparatus provided withan eddy current sensor operatively coupled to a wafer, in an embodimentdescribed herein;

FIG. 3 shows an eddy current signal, represented in arbitrary units as afunction of polishing time; and

FIG. 4 shows a collection of measures representing a variation of themean current collected at the very beginning of the CMP process, eachpoint being referred to the processing of a respective wafer by the samepolishing pad.

DETAILED DESCRIPTION

In an embodiment, schematically shown in FIG. 2, a CMP apparatus 10includes control electronics configured to calculate a thickness of aconductive layer 21 belonging to a wafer 20 during a planarization orpolishing or thickness-reduction process (in general, achemical-mechanical polishing “CMP” process). The CMP apparatus 10 canalso be employed to remove a bulk film and then stop, as in damascenecopper and tungsten polishing.

The CMP apparatus 10 includes a wafer carrier 14 facing a polishing pad16 that is arranged over a supporting plate 17. Both the wafer carrier14 and the plate 17 are controllable in rotation, as exemplified byarrows 19 a, 19 b, for performing the CMP process. The polishing pad 16is, for example, of polyurethane. A slurry delivery system (not shown),configured to provide a substantially uniform slurry layer onto thepolishing pad 16, may also be provided.

During a CMP process, the wafer 20 supported by the wafer carrier 14 ispressed against polishing pad 16 to process a surface of the wafer 20.In particular, the wafer 20 includes a substrate 13 having a top surface13 a on which the conductive film or layer (e.g., of Cu) 21 extends. Theconductive film or layer 21 is the target to be processed (e.g.,planarized) by the polishing pad 16.

The wafer carrier 14 is configured to support the wafer 20 at a bottomsurface 13 b of the substrate 13, during the CMP process. A coil-basededdy current sensor 18 is configured to induce an eddy current field inthe conductive layer 21, according to Faraday's induction law. Infurtherance of this aim, a sensing coil 18 a of the eddy current sensor18 is supplied with an alternating current (AC) generated by an ACcurrent generator (see, e.g., FIG. 1), thus causing a magnetic field toform around the coil 18 a. The eddy current sensor 18, which is arrangedin (or otherwise coupled to) the plate 17, is configured to detect asignal i_(EC) that is related to, or a function of, an actual thickness24 of the film 21. The signal i_(EC) is, in particular, the eddy currentfield that is induced in the layer 21 and that can be sensed by thesensor 18 as a change in the impedance of the coil 18 a of the sensor18; it is to be noted that the change of impedance is proportional tothe actual distance 25 between the sensor 18 and the top surface 13 a ofthe substrate 13 (not only to the thickness 24 of the layer 21). In theembodiment exemplified in FIG. 2, distance 25 is the sum, along a sameaxis Z, of the thickness 24 of layer 21 plus the thickness of thepolishing pad 16.

The eddy current sensor 18 generates at output a signal S_(EC), which isbased on signal i_(EC) and is a function of the thickness of the layer21 and of the thickness of the pad 16. As better explained later, in aninitial time window of the CMP process, the thickness of the layer 21 ofeach wafer being processed can be considered the same for each wafer(i.e., the thickness is such that the current is in saturation, so minorvariation the film thickness do not influence the eddy current signal).In that time window, the signal can be seen as a function of the padthickness only.

Eddy current sensors (ECS) allow for the contactless measuring of ametal film thickness in the full range of thicknesses normally utilizedin semiconductor manufacturing. The embodiment of FIG. 1 is thereforemerely exemplificative of the functioning of an eddy current sensor 18;other eddy current sensors may be employed.

A computer or processor, or any other processing circuitry 22 is incommunication with the sensor 18 and may be part of the CMP apparatus10, or operatively coupled to it. Alternatively, sensor 18 includes orintegrates the processor or the other processing circuitry 22, toperform certain computation on signal S_(EC), with the aim of evaluatingthe pad's consumption.

The CMP apparatus 10 is configured such that the signal S_(EC) is afunction of the thickness 24 of the film 21 and of the distance betweeneddy current sensor 18 and the film 21, which substantially correspondsto the thickness of the polishing pad 16. Both thicknesses of the film21 and of the polishing pad 16 are considered along a same axis (here,the Z axis). That is to say, a variation, during use, of the magneticfield sensed by the coil 18 a derives from both the thickness variationof the polishing pad 16 and the thickness variation of layer 21. In anaspect, information related to the actual thickness of the polishing pad16 is calculated or otherwise acquired by processing the signal S_(EC).In fact, the polishing pad 16 wears or erodes over time, causingvariation in the distance between the plate 17 and the wafer 20, whichinfluences the appropriated contribution to the total eddy currentsignal i_(EC).

During operation of the eddy current sensor 18, when an alternatingcurrent is caused to flow through the coil 18 a, which is set in closeproximity to the film 21, the electromagnetic field of the coil 18 ainduces eddy currents in the film 21. The phase of the eddy currents inturn affect the loading on the coil 18 a. Thus, the impedance of thecoil 18 a is impacted by the eddy currents. This impact is measured tosense the proximity of the film 21 to the sensor 18, as well as athickness of the film 21.

FIG. 3 represents an elaborated eddy current signal as function of theprocess time. With reference to region R1 of FIG. 3, at the beginning ofthe CMP process (in particular, from the start at 0 s until about 70 s,more in particular in the range 0-20 s), the variation of distance 25between eddy current sensor 18 and the substrate 13, as sensed by theeddy current sensor 18, is substantially constant. In fact, even thoughfilm 21 begins to be polished, the film 21 remains sufficiently thick(i.e., up to 3-6 micrometers) that the eddy current signal i_(EC)deriving from the film 21 is saturated for at least 70 seconds from thebeginning of processing. In other words, in region R1, the film 21 isthick, continuous and has a low resistivity, such that relatively strongeddy currents are generated in it and thus the elaborated current signalis in saturation. It has been found that the eddy current signals(fields) coming from the film 21 are saturated until the film 21 has athickness above about 2-2.5 μm. It follows that, in this situation,variation of the eddy current signal i_(EC) mainly derives from athickness reduction of the polishing pad 16.

As the film 21 erodes, the bulk portion of film 21 is thinned. As thefilm 21 thins, its sheet resistivity increases, and the eddy currentsbecome dampened and their contribution increases (region R2 in FIG. 3).

Eventually, the bulk portion of film 21 is removed (e.g., leavingconductive interconnects in the trenches between the patternedinsulating layer). At this point, the coupling between the conductiveportions in the substrate 13 (which are generally small and generallynon-continuous) and the sensing circuitry of sensor 18 reaches aminimum.

In region R3, a polishing endpoint is reached.

The above observations can be exploited during a plurality ofconsecutive CMP processes, wherein each process relates to the CMPtreatment of a respective wafer. Therefore, when a plurality of wafersare consecutively processed with the CMP apparatus 10, the signal S_(EC)during an initial timeframe, which corresponds to region R1 of FIG. 3(e.g., in the time window 0-70 s, as exemplified above) of each CMPprocess is monitored; potential drift of the signal S_(EC) from theexpected value is only (or at least mainly) caused by a variation of thepolishing pad's thickness.

FIG. 4 shows a collection of points, each one related to one respectivewafer 20 processed by the CMP apparatus 10. Each point represents acurrent signal value measured by the eddy current sensor 18 during saidinitial time frame (e.g., 0-70 s) of processing of one respective wafer20. Each point represents, therefore, a value of thickness 25 of FIG. 2(the ordinate axis of FIG. 4 is an arbitrary unit representing padthickness, while the abscissae axis is time). It is noted that, afterwafer processing with the same polishing pad 16, the value of thickness25, which is calculated from the signal provided by the eddy currentsensor 18, decreases. It is also noted that each newly processed wafer20 has a film 21 having, at the beginning of the respective CMP process,a similar thickness 24. Since the eddy current i_(EC) generated in theconductive film 21 is substantially constant (saturated) in theconsidered initial time frame, even minor variations in the metallicfilm thickness do not influence the output signal, and such variation ofthe current signal 25 is due to a variation of thickness of thepolishing pad 16.

By fitting the points of FIG. 4, one obtains a curve that drifts withtime (in particular, a descending curve); this drift depends by thevariation of the distance 25 (lift off) caused by reduction of thethickness of pad 16, which is consumed wafer after wafer.

Even though the signal S_(EC) (provided by the eddy current sensor 18 inthe initial time frame of processing of a wafer 20) is theoreticallyconstant during the initial time frame, small variations may anyway beobserved, caused by noise or other disturbing sources. Therefore, in anaspect, each point of FIG. 4 is the mean (average) value of therespective signal S_(EC) acquired for the respective wafer 20 in theinitial time frame of processing of such wafer 20.

It has been verified that, when the value of thickness 25 acquiredduring the above-mentioned time window (e.g., 0-70 s or 0-20 s) ofinitial processing of a certain wafer 20 is below a pre-set threshold,the polishing pad 16 it so undergo maintenance, in particular it is tobe replaced with a new polishing pad 16.

Such threshold can be different for different CMP apparatuses, subjectto the specific implementation and construction details of the CMPapparatus. Given a CMP apparatus, one can inspect the consumption statusof the polishing pad 16 and identify the value conveyed by signal S_(EC)outputted by the sensor 18 when the polishing pad 16 is completelyconsumed or erased (i.e., it is to be replaced with a new one); thevalue is identified during the time window of initial processing of thewafer, when the eddy current signal from layer 21 is saturated. In thisway, it is possible to pre-set a threshold that is specific for each CMPapparatus.

When the pre-set threshold is reached, a warning signal may beoptionally generated, signaling that the pad 16 should be replaced witha new one. In a non-limiting embodiment, in order not to risk a totalconsumption of the pad 16 during CMP processing, the warning signal canbe issued before the pre-set threshold has been reached; in anothernon-limiting embodiment, the pre-set threshold can be set at a valuehigher than the minimum value that corresponds to a complete warn orcomplete erase of the pad 16.

The advantages achieved by the disclosures herein are apparent from theabove description.

In particular, the above can be implemented in currently availableequipment, without the need for need for further (e.g., external)systems or devices to be used for checking the polishing pad's statusand consumption.

Finally, it is evident that modifications and variations may be made tothe present disclosure, without departing from the scope thereof, asdefined in the annexed claims.

For example, the system including the eddy current sensor 18 and theprocessor or processing circuitry 22 can be integral part of the CMPapparatus 10 or can be external to the CMP apparatus 10 and operativelycoupled to the CMP apparatus 10.

Moreover, the system can be employed in a variety of polishing systems.Either the polishing pad, or the carrier head, or both can move toprovide relative motion between the polishing surface and the substrate.The polishing pad can be a circular (or some other shape) pad secured tothe plate, a tape extending between supply and take-up rollers, or acontinuous belt. The polishing pad can be affixed on a plate,incrementally advanced over a plate between polishing operations, ordriven continuously over the plate during polishing. The pad can besecured to the plate during polishing, or there could be a fluid bearingbetween the plate and polishing pad during polishing. The polishing padcan be a standard (e.g., polyurethane with or without fillers) roughpad, a soft pad, or a fixed-abrasive pad.

1. A method for evaluating physical consumption of a polishing pad of aChemical Mechanical Polishing (CMP) apparatus, where the CMP apparatusis provided with an eddy current sensor that is configured to sense adistance using a substrate on which a layer to be processed by the CMPapparatus extends, the method comprising: generating, using the eddycurrent sensor, a magnetic field configured to induce eddy currents inthe layer during an initial time frame of CMP processing of the layer;acquiring, using the eddy current sensor during the initial time frame,an eddy current signal generated by the layer in response to themagnetic field; calculating an average value of the eddy current signalin the initial time frame; comparing the average value with a pre-setthreshold value; and determining a maintenance condition of thepolishing pad based on a result of the comparison.
 2. The method ofclaim 1, wherein the maintenance condition is a desire for replacementof the polishing pad.
 3. The method of claim 1, wherein the initial timeframe is such that the eddy current signal generated by the layer duringthe initial time frame is saturated.
 4. The method of claim 1, whereinthe initial time frame is such that the eddy current signal generated bythe layer during the initial time frame is saturated and substantiallytime-invariant.
 5. The method of claim 1, wherein the initial time frameis such that the eddy current signal generated by the layer during theinitial time frame is substantially time-invariant.
 6. The method ofclaim 1, wherein the initial time frame is between 0 and 70 seconds ofthe CMP processing of the layer.
 7. The method of claim 1, wherein theinitial time frame is between 0 and 20 seconds of the CMP processing ofthe layer.
 8. The method of claim 1, wherein the eddy current sensor isarranged with respect to the polishing pad in such a way that the eddycurrent signal is a function of both a thickness of the layer and athickness of the polishing pad.
 9. The method of claim 8, wherein theeddy current sensor is supported by, and moves with, a supporting platenover which the polishing pad is coupled.
 10. The method of claim 1,further comprising arranging the layer against the polishing pad beforethe initial time frame of processing of the layer by the CMP apparatus.11. A system for evaluating physical consumption of a polishing pad of aChemical Mechanical Polishing (CMP) apparatus, the system comprising: aneddy current sensor configured to sense a distance using a substrate onwhich a layer to be processed by the CMP apparatus extends, wherein theeddy current sensor is configured to generate, during an initial timeframe of CMP processing of the layer, a magnetic field configured toinduce eddy currents in the layer and to acquire, during the initialtime frame, an eddy current signal generated by the layer in response tothe magnetic field; and processing circuitry coupled to the eddy currentsensor and configured to: calculate an average value of the eddy currentsignal in the initial time frame, compare the average value with apre-set threshold value, and determine a maintenance condition of thepolishing pad based on a result of the comparison.
 12. The system ofclaim 11, wherein the maintenance condition is a replacement of thepolishing pad.
 13. The system of claim 11, wherein the initial timeframe is such that the eddy current signal generated by the layer duringthe initial time frame is one of saturated and/or substantiallytime-invariant.
 14. The system of claim 11, wherein the initial timeframe is between 0 and 70 seconds of said CMP processing of the layer.15. The system of claim 11, wherein the initial time frame is between 0and 20 seconds of said CMP processing of the layer.
 16. The system ofclaim 11, wherein the eddy current sensor is arranged with respect tothe polishing pad in such a way that the eddy current signal is afunction of a thickness of the layer plus a thickness of the polishingpad.
 17. The system of claim 11, wherein the eddy current sensor issupported by, and moves with, a supporting plate over which thepolishing pad is coupled.
 18. A CMP apparatus comprising a systemaccording to claim
 11. 19. A chemical machine polishing (CMP) apparatus,comprising: a wafer carrier facing a polishing pad, the polishing padbeing disposed over a supporting plate; a wafer carried by the wafercarrier, the wafer having a top surface over which a conductive layerextends; and an eddy current sensing device configured to induce an eddycurrent field in the conductive layer, and detect a change in impedanceof a coil of the eddy current sensing device; wherein the eddy currentsensing device is further configured to generate, from the change inimpedance, an output signal based upon a thickness of the conductivelayer and a distance between the eddy current sensing device and theconductive layer, the output signal corresponding to a thickness of thepolishing pad.
 20. The CMP apparatus of claim 19, wherein the eddycurrent sensing device is supported by, and moves with, the supportingplate.
 21. The CMP apparatus of claim 19, wherein the eddy currentsensing device induces the eddy current field in the conductive layer byan alternating current flowing through the coil.
 22. The CMP apparatusof claim 19, wherein the change in the impedance indicates that athickness of the polishing pad has reduced below a threshold level.